Heretofore, graphite has been commonly used as a cathode for electrolysis of acidic electrolytes comprising hydrochloric acid, sulfuric acid, organic acids or mixtures threof. Although graphite is inexpensive and has superior corrosion resistance and hydrogen brittle resistance, it has disadvantages in that the hydrogen generation potential is high, the electrical conductivity is relatively low, and the mechanical strength and workability are poor. To overcome these problems, various attempts have been made: for example, East German Pat. No. 62,308 discloses a method of plasma flame spraying tungsten carbide or titanium carbide on graphite to produce a cathode having a low hydrogen overvoltage and to reduce the electrolytic voltage. This method, however, fails to overcome the problems involved in using graphite as a cathode substrate.
Various cathodes are also known, which comprise a metallic substrate and a coating layer on the substrate, the coating layer being composed of a substance having a low hydrogen overvoltage. For example, Japanese Patent Application (OPI) No. 32832/77 (the term "OPI" as used herein means a "published unexamined Japanese patent application") discloses a cathode for the electrolysis of chlorine/alkalis which is prepared by spray coating an iron-base metal substrate with a powdery metal having a low hydrogen overvoltage. These cathodes have improved mechanical strength and workability because metals are used as their substrates. However, use of these cathodes in the electrolysis of chlorine/alkalis wherein the cathode electrolyte is alkaline is intended. When these are used as cathodes for the electrolysis of the above-described acid solution, they suffer from various disadvantages in that they have insufficient corrosion resistance and are not durable for practical use.
A cathode for the electrolysis of acid solutions comprising an electrically conductive metallic substrate, a sprayed coating layer of a cathode active substance containing W, WC or a mixture thereof, and an impregnated coating layer of an acid-resistant fluorine-based resin, where the impregnated coating layer is provided on the external surface of the sprayed coating layer (see Japanese Patent Application No. 148698/81 (corresponding to U.S. patent application Ser. No. 416,512, filed Sept. 9, 1982 abandoned) has already been developed.